There has been a considerable amount of interest in silicon carbide (Sic) semiconductors recently due to breakthroughs which have made single crystals attainable, for example, as described in the article entitled "Growth and Characterization of Cubic SiC Single Crystal Films on Silicon", by J. A. Powell, L. G. Matus, and M. A. Kuczmarski in Journal of Electrochemical Society, Vol. 134, No. 6, pp. 1558-1565, June 1987. SiC has a wide bandgap (2.2 eV for .beta.-SiC), a high melting point (2800 degrees C.) and a large thermal conductivity, making it an excellent semiconductor for high temperature applications. Recent research has shown that .beta.-SiC has excellent properties for use as a sensing element for piezoresistive transducers. Such high temperature SiC transducers are described in the related U.S. patent application Ser. No. 07/694,490, entitled "High Temperature Transducers and Methods of Fabricating The Same Employing Silicon Carbide", filed on May 2, 1991, and assigned to Kulite Semiconductor Products, Inc., in common with the present application, which is incorporated by reference herein. This patent addresses two issues in the fabrication of SiC pressure sensors and other devices, namely, etching and metallization.
There have been previous attempts to etch SiC using reactive ion etching, plasma etching, and photoelectrochemical etching. For example, reactive ion and plasma etching of SiC are described in the article entitled "Dry Etching of .beta.-SiC in CF.sub.4 and CF.sub.4 +O.sub.2 Mixtures", by J. W. Palmour, R. F. Davis, T. M. Wallett, and K. B. Bhasin, in Journal of Vacuum Science Technology, Vol. 4, No. 3, published May/June 1986. Photoelectrochemical etching of n-type .beta.-SiC under ultraviolet (UV) illumination in hydrofluoric acid (HF) solution is described in a previous paper by the authors of the present invention, entitled "Surface Micromachining of .beta.-SiC Using Laser Controlled Photoelectrochemical Etching" in "Amorphous and Crystalline SiC III", to be published by Springer/Verlag in 1991. However, none of these etching processes employed any selective etching of different conductivity types of SiC. Conductivity selective etching is extremely important for sensor fabrication. The use of conductivity selective etch stop allows for the precise fabrication of thin membranes. These membranes can act as the force collector in an integrated transducer.
Techniques have been proposed for selective etching of a semiconductor layer using an epitaxially grown stop layer or ion-implanted barrier layer that does not dissolve in a selected etching solution, and resistivity gradient limiting etching through different rates of anodically dissolving an n-type silicon layer on a p-type silicon substrate. See, for example, the article entitled "The Fabrication of Thin, Freestanding, Single-Crystal, Semiconductor Membranes", by K. C. Lee, in Journal of Electrochemical Society, Vol. 137, No. 8, August 1990. However, these prior techniques have not been applied to selective etching of different conductivity types of SiC.